The present invention relates to a technique for changing over optical systems having different magnification powers upon alteration of the size of original in an image forming apparatus in which the original and a film are both disposed in the stationary state so that the original is imaged on a surface of the film and which apparatus is arranged such that a center portion of an original supporting stage is projected substantially onto a center portion of the film.
In the image forming apparatus in which an original is placed on a supporting stage and the image thereof is formed on a light sensitive medium such as film or the like, the original has to be so positioned that the center thereof coincides with that of the image regardless of variation in the size of the original. Otherwise, a portion of the original may be excluded from the image. Further, when originals of different sizes are to be taken on a film of a same size, there are required a plurality of lenses having different magnification powers in accordance with the sizes of the originals. Thus, operator must position an original and select the lens in accordance with the size of the original before photographing it.
However, the operator may neglect the above procedure and select a wrong lens, resulting in wasteful photographing to disadvantage.
In view of the drawback of the prior art described above, it is an object of the present invention to provide a magnification power varying mechanism for automatically changing the taking lens to the one having a requisite magnification power upon positioning of an original.